Move reinitializeCompositing and restartKWin into the Compositor

The DBus signal which causes KWin to reinitialize the Compositor
is moved into the Compositor as everything can be handled from
there as well. This comes together with moving the restartKWin
functionality into the Compositor as it is only relevant there.
Restart will only happen if the wrong Qt graphicssystem is used
for the chosen compositing backend.